1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the same. The present invention is suitable for a color image forming apparatus such as a laser beam printer, a digital copying machine, or a multi-function printer, which adopts an electrophotography process.
2. Description of the Related Art
An image forming apparatus such as a digital copying machine or a laser beam printer (LBP) has conventionally used an optical scanning apparatus for forming an image.
The optical scanning apparatus of this type uses, as a unit for miniaturizing the entire apparatus, opposed scanning units which perform scanning with multiple light beams by means of the same optical deflector, and guides the light beams to different surfaces to be scanned by imaging optical systems disposed on both sides of the optical deflector.
Various optical scanning apparatus that use such opposed scanning units have conventionally been presented (refer to Japanese Patent Application Laid-Open Nos. 2003-202512 and 2005-004050).
Japanese Patent Application Laid-Open Nos. 2003-202512 and 2005-004050 discuss optical scanning apparatus which deflect light beams in symmetrical directions with respect to a shared optical deflector (rotational polygon mirror) for scanning.
FIG. 20 is a main portion schematic diagram of the optical scanning apparatus of Japanese Patent Application Laid-Open No. 2003-202512 shown in FIG. 1. The optical scanning apparatus of the type that share the optical deflector shown in FIG. 20 have the following problem when one optical scanning apparatus (first scanner SK11) forms an image. An undesirable light (ghost light) Rf reflected on a lens surface 91a of a first imaging lens 91 of a first imaging lens system SL11 is transmitted through an imaging lens system SL12 of the other optical scanning apparatus (second scanner SK12) to enter a photosensitive drum surface 99.
Similarly, there exists an undesirable light reflected on a lens surface of a first imaging lens 92 of the second imaging lens system SL12 to enter the first imaging lens system SL11.
Depositing an anti-reflection film on the lens surface enables reduction of undesirable lights. However, in the case of a plastic lens often used recently for reducing weight and costs of the optical scanning apparatus, deposition of an anti-reflection film is difficult. Thus, there is a demand for a method of reducing undesirable lights by using means other than an anti-reflection film.
To easily realize an optical scanning apparatus which forms a four-color image by using one optical deflector, a sub-scanning in-section oblique incident optical system has been employed which obliquely input light beams in a sub-scanning section to a deflecting surface of the optical deflector.
FIG. 21 is a perspective diagram of a main portion around the rotational polygon mirror 201 of Japanese Patent Application Laid-Open No. 2005-004050 shown in FIG. 1.
The opposed scanning units and the sub-scanning in-section oblique incident optical system discussed in Japanese Patent Application Laid-Open No. 2005-004050 include an undesirable-light blocking member 206 disposed to prevent incidence of undesirable lights to imaging optical elements 202a and 202b. Specifically, an undesirable light spatially separated from real light beams for image formation in the sub-scanning section is blocked by using the undesirable-light blocking member 206.
The undesirable-light blocking member 206 prevents reaching of any undesirable lights to the surface to be scanned (not shown).
Recently, in order to miniaturize the optical scanning apparatus, optical elements of the optical scanning apparatus have been densely arranged, resulting in a complicated arrangement. Inadequate assembling accuracy of a box (casing) to mount the optical elements has created a tendency toward lower mounting accuracy of the optical elements.
In order to further miniaturize the optical scanning apparatus, the optical box and the undesirable-light blocking member have been integrally formed.
Typically, an undesirable light enters the undesirable-light blocking member to be blocked. However, if an optical element (optical member) is shifted from a desired position, an undesirable light reflected on a surface of the optical element strays from a predicted optical path. Thus, without entering the undesirable-light blocking member, the undesirable light may reach the surface to be scanned which is located ahead on the optical path. In consequence, during image formation, image defects such as density fluctuations of an image may occur.
In Japanese Patent Application Laid-Open No. 2005-004050, in FIG. 21, for example, it is presumed that a mounting error occurs in the imaging optical element 202b so that the imaging optical element 202b is tilted in the sub-scanning section. In this case, an undesirable light reflected on an incident surface of the imaging optical element 202b passes through an opening 206a of the undesirable-light blocking member 206. The undesirable light that has passed through the opening 206a enters the imaging optical element 202a to reach the surface to be scanned (not shown). As a result, during image formation, image defects such as density fluctuations of an image occur.
When the imaging optical element is disposed in a position near the surface to be scanned, an undesirable light reflected on an incident surface of the imaging optical element passes through the position near an optical reference axis in substantially parallel to the optical reference axis.
Note that the optical reference axis is an axis passing through an incident point to a defecting surface of a principal beam (principal ray) of light beams and vertical to the deflecting surface in the sub-scanning section when the principal beam of the light beams emitted from the incident optical system is deflected on the deflecting surface of the optical deflector enters the center of the surface to be scanned.
Thus, in order to block unexpected fluctuations of an undesirable light or an undesirable light reflected on another optical element, an opening (opening portion) of the undesirable-light blocking member has to be narrowed as much as possible.
However, the excessively narrow opening of the undesirable-light blocking member causes blocking of even a part of real light beams for forming a latent image on the photosensitive drum surface.